Re: Can we have 'unless' in a negative sentence?
As a matter of style, I don't like them much because it makes the reader have to untangle the thought. Do this.. unless I haven't done that. So if I have done that, then I do this. Hmm.
That's why I suspect you won't see it in writing as much.
However, I suspect in speech we use it much more often, usually because we realize that there might be a situation in which something we've just said won't apply in some situations (like ems's camera example) so we amend our original statement.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.